The photographic industry has long recognized the need to provide photographic film and paper with antistatic protection to prevent the accumulation of static charges during manufacture and use. Static charges can cause irregular fog patterns in photographic emulsions and various coating imperfections such as mottle patterns and repellency spots. Such charges also attract dirt and dust to the photographic element surface which may result in the formation of "pinholes" in processed films as well as a variety of handling and conveyance problems.
To prevent the problems arising from an accumulation of static charges, it is conventional practice to provide an antistatic layer (i.e., a conductive layer) in a photographic element. A wide variety of antistatic layers are known for use in photographic elements. For example, U.S. Pat. No. 3,033,679 discloses an antistatic layer comprised of an alkali metal salt of a copolymer of styrene and styrylundecanoic acid. Photographic films having a metal halide, such as sodium chloride or potassium chloride, as the conducting material in a hardened polyvinyl alcohol binder are described in U.S. Pat. No. 3,437,484. In U.S. Pat. No. 3,525,621, the antistatic layer is comprised of colloidal silica and an organic antistatic agent such as an alkali metal salt of an alkylaryl polyether sulfonate, an alkali metal salt of an arylsulfonic acid, or an alkali metal salt of a polymeric carboxylic acid. An antistatic layer comprised of an anionic film forming polyelectrolyte, colloidal silica, and a polyalkylene oxide is disclosed in U.S. Pat. No. 3,630,740 while U.S. Pat. No. 3,681,070 describes a copolymer of styrene and styrene sulfonic acid as an antistatic agent. U.S. Pat. No. 4,542,095 describes antistatic compositions comprising a binder, a nonionic surface-active polymer having polymerized alkylene oxide monomers, and an alkali metal salt. In U.S. Pat. No. 4,916,011, an antistatic layer comprising a styrene sulfonate- maleic acid copolymer, a latex binder, and a alkyl-substituted trifunctional aziridine crosslinking agent are disclosed.
It is known to prepare an antistatic layer from a composition comprising metal oxides, and particularly vanadium pentoxide as described, for example, in Guestaux, U.S. Pat. No. 4,203,769. Antistatic layers containing vanadium pentoxide provide excellent protection against static and are highly advantageous in that they have excellent transparency and their performance is not significantly affected by changes in humidity. It is also known to provide metal oxide layers, including vanadium pentoxide antistatic layers, with a protective overcoat layer such as a layer of a cellulosic material to provide abrasion protection and/or enhance frictional characteristics.
In some types of photographic elements, the antistatic layer is located on the side of the support opposite to the image- forming layers and it is not necessary for there to be any functional layers overlying the antistatic layer except for the optional inclusion of a protective overcoat layer. Vanadium pentoxide antistatic layers which may contain a polymeric binder are effectively employed with such elements and may serve as the outermost layer, or, optionally, may be provided with an overlying cellulosic layer which serves as a protective abrasion-resistant topcoat layer. In other types of photographic elements, however, the antistatic layer must function as both a subbing layer and an antistatic layer. Thus, for example, many photographic elements contain a gelatin-containing pelloid layer on the side of the support opposite to the image-forming layers in order to control curl. Such elements commonly contain a layer underlying the curl control layer which acts both as a subbing layer and an antistatic layer. Other photographic elements such as X-ray films are coated with silver halide emulsion layers on both sides and are provided with a layer which functions as both a subbing layer and antistatic layer underlying each silver halide emulsion layer. Serious difficulties are encountered when vanadium pentoxide antistatic layers are used as subbing layers. For example, silver halide emulsion layers and curl control layers do not adhere well to vanadium pentoxide antistatic layers and, as a consequence, delamination can occur. Vanadium pentoxide can diffuse from the subbing layer through the overlying emulsion layer or curl control layer into processing solutions thereby resulting in diminution or loss of the desired antistatic protection after the film is processed.
U.S. Pat. No. 5,006,451 discloses the application of a latex polymer barrier layer over a vanadium pentoxide antistatic subbing layer to prevent the loss of antistatic properties during processing and provide good adherence to subsequently applied hydrophilic colloid layers such as, for example, curl control layers. However, such latex barrier layers require use of significant quantities of high boiling organic solvent "coalescing aids" which tend to volatilize on drying resulting in coating imperfections, and lack of uniformity and adhesion of subsequently applied layers, as well as conveyance problems.
To insure coalescence of the latex polymer from its particulate latex form to a coherent film capable of acting as a barrier layer during the extremely short drying times used in high speed film support manufacture, significant concentrations of high boiling organic solvent "coalescing aids" are used in the latex formulation. Coalescing aids lower the glass transition of the latex polymer during drying, causing the latex particles to flow and form a film. While some of the coalescing aid remains permanently in the latex film, such materials also partially volatilize when the barrier coating is dried. Subsequent condensation of volatilized coalescing aid in cooler areas of the coating apparatus causes coating imperfections and conveyance problems. In addition, as a latex coalesces in the presence of coalescing aids, it is well known that some of the coalescing aid exudes to the surface of the coating. This surface layer of exuded high boiling organic compound (coalescing aid) can adversely effect the uniformity and adherence of subsequently applied layers such as photographic emulsions or curl control layers.
Further, latex barrier polymers in aqueous formulations, with or without the use of coalescing aids, are low viscosity liquids which do not increase in viscosity until nearly all of the water evaporates during the drying process. As the coating dries using conventional high temperature air impingement, the uniformity of the low viscosity liquid coating is disturbed, resulting in a non-uniform "mottled" layer by the time the coating is fully dried. In a photographic element, such non- uniformity causes serious problems, particularly because the mottle pattern can transfer to photographic emulsion or curl control layers when they are applied over the barrier layer.
Accordingly, an antistatic photographic film support comprising a vanadium pentoxide antistatic layer and a barrier layer therefor which does not require coalescing aids and does not exhibit drying-induced mottle patterns is desired.